How to invert colors of an image in pygame?

I have a pygame Surface and would like to invert the colors. Is there any way quicker & more pythonic than this? It's rather slow.

I'm aware that subtracting the value from 255 isn't the only definition of an "inverted color," but it's what I want for now.

I'm surprised that pygame doesn't have something like this built in!

Thanks for your help!

import pygame

def invertImg(img):
    """Inverts the colors of a pygame Screen"""

    img.lock()

    for x in range(img.get_width()):
        for y in range(img.get_height()):
            RGBA = img.get_at((x,y))
            for i in range(3):
                # Invert RGB, but not Alpha
                RGBA[i] = 255 - RGBA[i]
            img.set_at((x,y),RGBA)

    img.unlock()